Sputter deposition, also known as sputter coating, is a technique for depositing thin films of desired materials on a substrate such as, for example, a glass panel for a flat panel display, a magnetic disk for a hard disk drive or a semiconductor wafer. In general, ions of an inert gas from a plasma are accelerated toward a target of the material to be deposited. Free atoms of the target material are expelled when the ions collide with the target. A portion of the free atoms are collected on the surface of the substrate and form a thin film.
One well known sputtering technique is magnetron sputtering. Magnetron sputtering uses a magnetic field to concentrate the sputtering action. Magnets are positioned behind the target, and magnetic field lines penetrate the target and form arcs over its surface. The magnetic field helps to confine electrons in an area near the surface of the target. The resulting increased concentration of electrons produces a high density of ions and enhances the efficiency of the sputtering process.
Both fixed and movable magnet structures have been utilized in magnetron sputtering. In one prior art sputtering system utilizing a moving magnet, the target is circular and the magnet structure rotates with respect to the center of the target. In a second prior art sputtering system utilizing a moving magnet, the target is rectangular or square and the magnet structure is scanned along a linear path with respect to the target. In a third prior art sputtering system, the target is rectangular and the substrate is moved in a plane parallel to the surface of the target during sputtering. The second type of sputtering system is disclosed, for example, in U.S. Pat. No. 5,382,344 issued Jan. 17, 1995 to Hosokawa et al and U.S. Pat. No. 5,565,071 issued Oct. 15, 1996 to Demaray et al.
Prior art sputtering systems which utilize a rectangular or square target and a linearly scanned magnet structure have a number of problems and disadvantages. The requirement for large substrates has led to the need for large target assemblies. Such target assemblies usually include target tiles solder bonded to a single water-cooled backing plate. The bonded target tiles and the backing plate form a bimetal structure which bows permanently during the bonding operation in response to temperature variations because of differing rates of thermal expansion. Prior art attempts to minimize bowing have utilized target tile segments, expansion gaps between target tile segments, low melting point solder and backing plate materials that have a closer match in expansion/contraction coefficient with the target material. Bowing of the target can be significant, up to 0.060 inch for a 17 inch target using indium tin oxide tiles and a copper backing plate. Bowing of the target assembly produces variations in target-to-substrate distance and may compromise some other design parameter that is critical to the magnetron source operation.
A sputter cathode wherein target segments are mounted on a mounting plate by locating studs is disclosed in U.S. Pat. No. 5,536,380 issued Jul. 16, 1996 to Ocker et al. Each target segment includes a target backplate and a target bonded to the backplate.
Linear scan sputtering systems typically utilize a magnet assembly which produces a closed-loop plasma in the shape of an elongated oval, often referred to as a racetrack-shaped plasma. The magnet assembly is scanned with respect to the target in a direction perpendicular to the long dimension of the racetrack-shaped plasma. The resulting erosion of the target tends to be greatest at the ends of the plasma, thus producing erosion grooves along each edge of the target parallel to the scan direction. To avoid contaminating the substrate, sputtering must be stopped before the erosion pattern has consumed the full thickness of the target material at any point. The target must be replaced when the erosion at any point approaches a substantial fraction of the target's initial thickness. Thus, in a given production process, only a certain number of substrates can be coated from one target. By making the erosion of the target more uniform, a larger percentage of the target material may be utilized before it is necessary to replace the target.
The linear scanning sputter source typically has a large rectangular or square target attached to a water-cooled backing plate. This structure operates at cathode potential, typically several hundred volts negative with respect to ground. On the backing plate side of the target, a racetrack-shaped magnet assembly creates a magnetic field at the target surface that is sufficient to sustain a plasma when the appropriate electric field is provided. The electric field is created by operating the target structure at cathode potential in a chamber held at ground potential. The electric field at the target surface is influenced by the size and shape of the chamber and by the characteristics of the plasma which is formed. The combination of magnetic and electric fields in a near vacuum causes the sputtering action to occur.
Depositional thickness uniformity is an important requirement of sputtering systems. For large area substrates, uniform deposition is achieved by using large targets and linearly scanning the magnet assembly with respect to the target, producing uniform erosion of the target surface and uniform coating of the substrate. However, the inherent geometry of a large area sputtering system produces effects which limit thickness uniformity.
The magnet assembly is linearly scanned parallel to the target surface and perpendicular to the long dimension of the magnet assembly. This motion causes the plasma, which is formed a few millimeters away from the target surface, to sweep across the target surface. The geometry of the sputtering chamber affects the depositional thickness uniformity. In particular, when the chamber walls are relatively close to the ends of the racetrack-shaped plasma, the deposited films are relatively thin in the center of the substrate and relatively thick at the edges. It is desirable to provide a sputtering system which achieves depositional thickness uniformity.